When a component has a characteristic with a non-linear dependence on another characteristic it forms in many circuit applications a disadvantage which must be compensated for or minimized in some way. As an example of such a component we discuss in the following a capacitance diode being part of the frequency synthesizer in a radio telephone and having a typical characteristic curve as shown in FIG. 1.
FIG. 1 shows the variation 1 in capacitance of a capacitance diode as a function of the applied voltage. The capacitance diode provides the capacitance in the resonator in a voltage controlled oscillator (VCO). The capacitance value is controlled by a d.c. voltage, whereby the frequency of the resonator varies in relation to the capacitance. The capacitance of the capacitance diode has a non-linear dependence on the controlling d.c. voltage, which causes a deviation variation between different channels in the radiotelephone. At least two methods are commonly used to compensate for this non-linearity.
The first compensation method can be used if a sufficiently high battery voltage, over 9 V, is available in the device such as a mobile phone. When this battery voltage can be utilized so that the non-linear component is caused to operate in its more linear region, i.e. the region above 3 V according to the characteristic in FIG. 1.
Another widely used compensation method, is called programmable correction, which is predominant in radio telephones, particularly in devices such as handheld phones with a low battery voltage, below 7.2 V. Then the non-linear component must operate on the non-linear region of the capacitance/voltage-characteristic. Then the component causes a deviation variation between different channels in the radio telephone. It is well known to use programmable correction of this deviation variation, because the variation on each channel is known. The curve 2 in FIG. 2 shows the deviation variation for the channels in a radiotelephone as a function of the control voltage Vohj. The correct value of the deviation is indicated as the percentage, in relation to which the variation is shown. With programmable correction it ks possible to extend the control voltage region downwards, whereby a smaller VCO-factor is obtained.
An alternative to the use of programmable correction is to increase the control voltage above 3.0 V on the lowest channel, whereby the control voltage of the VCO can vary from 3 V to the regulated supply voltage. Then there is the problem of the small difference between the control voltage of the lowest channel and the regulated supply voltage of the synthesizer, which is typically 6 V. The whole frequency band of the oscillator has to be generated with the voltage difference, so that the method requires a large VCO-factor, over 10 MHz/V, and great accuracy in manufacture, so that the channel band control voltages can be adjusted between 3.0 V and the regulated supply voltage. This requires a margin of at least 0.5 V between the control voltage and the regulated supply voltage.
Thus the disadvantages of known compensation methods are the need to use a large VCO-factor, whereby phase noise and a noisy supply voltage cause inconveniences, and the great accuracy requirements on the production. There is also the disadvantage of a long setting time of the synthesizer, from the POWER OFF state to the desired channel.